The present invention relates to a series of new dithiolan derivatives having an excellent ability to enhance the activity of glutathione reductase. The invention also provides a process for preparing these compounds and methods and compositions using them.
Glutathione is found throughout the tissues of the living body, is a major reducing agent in cells, and plays a very important role in the oxidation-reduction metabolic processes. In particular, reduced glutathione (GSH), thanks to the presence of a thiol group, plays a key role in various cellular defence and repair mechanisms. Glutathione peroxidase catalyses the reactions involved in these mechanisms, and is an important enzyme in the antioxidant system, wherein perocides (e.g. hydrogen peroxide, lipid peroxides and so on) are reduced by GSH. On the other hand, glutathione reductase is an enzyme which reduces oxidized glutathione (oxidized-type glutathione; GSSG) in the presence of NADPH to regenerate GSH.
The antioxidant system comprising these materials and enzymes protects cells from the harmful effects of oxidising materials (e.g. above described peroxides, free radicals and so on). Oxidative stress occurs when the balance between oxidising materials and the antioxidant mechanisms is shifted in favor of the former [J. Appl. Physiol. 1996 November, 81(5), 2199-2202]. It has been reported that oxidative stress is associated with various diseases, such as coronary heart disease, cataracts, pulmonary diseases (e.g., idiopathic pulmonary fibrosis, adult respiratory distress syndrome, emphysema, asthma, bronchopulmonary dysplasia and interstitial pulmonary fibrosis), chronic renal failure, disorders of the nervous system including the peripheral nervous system and the central nervous system (e.g. Parkinson's disease, schizophrenia, Alzheimer's disease, epilepsy, amyotrophic lateral sclerosis and cerebral ischemia), gastric ulcers, diabetes, hepatocyte necrosis and apoptosis including ethanol-induced hepatopathy, viral disease (including influenza, hepatitis B and HIV), and colorectal cancer [J. Appl. Physiol. 1996 November, 81(5), 2199-2202; Free Radical Biology & Medicine, Vol. 21 No. 6, 845-853 (1996); Free Radical Biology & Medicine, Vol. 20 No. 7, 925-931 (1996); Gastroenterology, 112, 855-863 (1997); Free Radical Biology & Medicine, Vol. 34, 161-165 (1996); Lancet, 338, 215-216 (1991); Diabetologia, 39, 357-363 (1996); Eur. J. Cancer., 1996 January, 32A(1), 30-38; Am. J. Med., 1991 September 30. 91(3c), 95s-105s; Alcohol. Clin. Exp. Res., 1996 December 20(9 Suppl), 340A-346A; Free Radical Biology & Medicine, Vol. 21 No. 5, 641-649 (1996); Pharmacol. Toxicol., 1997 April, 80(4), 159-166; Cell. Mol. Biol. (Noisy-le-grand) 1996 February, 42(1), 17-26; Prostaglandins. Leukot. Essent. Fatty Acids, 1996 August, 55(1-2), 33-43; FASEB J., 1995 September, 9(12), 1173-1182].
In addition to the above, oxidative stress is thought to be a factor in Down's syndrome, nephritis, pancreatitis, dermatitis, fatigue, rheumatism, various malformations (e.g. Duchenne muscular dystrophy, Becker dystrophy, Dubin-Johnson-Spring syndrome, favism and so on), Fanconi's anemia, canceration and metastases, septicemia, enhanced permeability of the blood vessels, leukocyte adherence, retinopathy of prematurity, siderosis, toxic effects of medicines (e.g. carcinostatics including platinum chelate, antibiotics, antiparasitics, paraquat, carbon tetrachloride and halothane) and radiogenic damages [Yoshihiko Oyanagi, Superoxide dismutase and agents controlling active oxygen species].
In WO94/12527, it is disclosed that compounds which enhance the synthesis of endogenous GSH are suitable for human therapy, in particular for the treatment of various diseases induced by glutathione deficiency, such as the pathological states related to oxidative tissue damage, in particular when resulting from an excess of free radicals. Some examples of such diseases are: intracellular oxidative state disequilibrium following alcohol abuse, exposure to xenobiotic agents, damage caused by radiation, hepatic diseases, intoxication from drugs and chemical agents, poisoning by heavy metals, physiological brain ageing (e.g. Parkinson's disease), brain degeneration due to decreased glutathione levels caused by altered antioxidant defence mechanisms, such as acute and chronic neurodegenerative diseases (e.g., acute pathologies such as: acute ischemic states, in particular cerebral ictus, hypoglycemia, and epileptic attacks; chronic pathologies such as: amyotrophic lateral sclerosis, Alzheimer's disease, Huntington's chorea), diseases related to altered functionality of the immune system, in particular resulting from tumour immunotherapy, and infertility, in particular male infertility. It is also disclosed that the compounds are suitable for organ reperfusion following ischemic events mainly imputable to free radicals.
Furthermore, in Japanese patent publication Kokai Showa 64-26516, it is disclosed that a compound which increases glutathione levels is useful for the treatment and prevention of various diseases including cataracts, hepatic disorders, nephritic disorders.
At this time, lipoic acid (thioctic acid), which has dithiolan ring in its molecule, is known to influence the biosynthesis and regeneration of reduced glutathione [I. Maitra et al., Free Radical Biology & Medicine, Vol. 18 No. 4, 823-829 (1995)]. In this literature, it is reported that the total glutathhione (oxidized and reduced glutathione) level is decreased by administering buthionine sulfoximine (BSO), which is an inhibitor of glutathione synthetase, to newborn rats, that the decrease is prevented by administering lipoic acid together with BSO, and that cataract formation is suppressed. In addition, the literature describes a test on the effects on glutathione reductase achieved by administering only BSO or by administering both BSO and lipoic acid. Considering these results, it is understood that the activity of glutathione reductase does not change when BSO is simply administered by itself, and that the activity of glutathione reductase also does not increase when lipoic acid is administered in addition to BSO.
It can, therefore, be deduced from this literature that the total glutathione level will be increased and that disorders can be treated when lipoic acid is administered to a patient who is suffering from a disease caused by a deficiency of glutathione synthesis, but lipoic acid is not thought to provide sufficient effect against diseases which occur in spite of enough glutathione synthesis since it is understood not to increase glutathione reductase activity.
On the contrary, if the activity of glutathione reductase can be increased, then whether glutathione synthesis is or is not adequate, diseases which occur in spite of enough glutathione synthesis and which are caused by oxidative stress can be prevented or treated since the supply of reduced glutathione is increased.
Furthermore, in general, in the case of ophthalmologic diseases, such as cataracts, topical application to the eyes is preferred to oral administration. However, since lipoic acid is a powerful stimulant, it is impossible to administer it to the eyes.
We have now discovered a series of dithiolan derivatives, which have the ability to cause a significant increase in the activity of glutathione reductase and which also remove peroxides. Moreover, the compounds of the present invention are less stimulating to the eyes than lipoic acid and similar known compounds are thus especially suitable for topical application.
For the avoidance of doubt, the compounds of the present invention are named following the IUPAC Rules, using, as appropriate, lipoic acid (also known as thioctic acid) as the parent compound. This compound has the formula: ##STR2##
It is an object of the present invention to provide a series of new dithiolan derivatives.
It is a further and more specific object of the present invention to provide such compounds which increase the activity of glutathione reductase.